The invention relates to a hydrostatic gear machine comprising a pair of gear wheels meshing with one another, one of said gear wheels being an internally toothed annular gear wheel and the other an externally toothed pinion, the tooth profile of at least one of said wheels being at least substantially trochoidal.
The expression "substantially trochoidal gearwheels" is intended herein to cover not only a pair of gearwheels of which at least one wheel has a trochoidal toothing, in particular, a cycloidal toothing, but also a pair of gearwheels of which at least one wheel has a circularly arcuate toothing, since in the case of the latter the profile of the flanks of the teeth is very similar to that of a trochoidal toothing.
The invention is particularly concerned with a pair of gearwheels of this kind wherein the difference in the number of teeth of the annular wheel on the one hand and the pinion on the other is very small but is greater than one.
Rotary fluid-displacing machines having pairs of gearwheels of this kind offer the advantages of a very compact construction, a small space-requirement, good delivery and suction capacities, low pulsation during fluid delivery, low tooth-engagement frequencies, reduced noise when operating, and the possibility of providing the pinion with a very thick shaft or forming therein a very large opening through which the shaft passes. They are particularly suitable as feed pumps which can be directly fitted, in an economical manner, on the main shafts of considerably larger engines or motors so as to form an integral part thereof.
A pair of trochoidal gearwheels of this kind is disclosed for example in German Offenlegungsschriften Nos. 2,024,339 2,041,483 and 2,318,753 (equivalent to U.S. Pat. No. 3,907,470), and in U.S. Pat. No. 3,782,040.
Advantageously, the toothing of the annular wheel in such gear machines is initially determined with a view to the width of the root of the tooth being approximately equal to twice the width of the gap between teeth at the root circle. A provisional form of tooth is then established in the form of a triangle, the height of which is approximately three-fifths of the width of the root of the tooth. The shorter sides forming the flanks of the tooth are then uniformly outwardly curved to give a convex form, so that a tooth profile is created as shown, for example, in German Pat. OS No. 2,024,339, wherein the apices of the tooth profiles that are determined in this manner are at a distance of approximately one-twentieth of the width of the root from the corresponding sides of the triangle.
With pairs of trochoidal gear wheels of this kind, the degree of overlapping is very much greater than one, and is generally a multiple of one, since the line of engagement winds around the pinion. If the shapes of the flanks of both gearwheels were obtained in an absolutely faultless manner and if the gearing operated absolutely without clearance, there would be no objection to this extended tooth engagement having a large overlap.
In practice, however, such precision and operating conditions can never be achieved. Errors in the flanks of the teeth, in revolution and in pitch, as well as errors in the direction of the flanks, and the fact that the distance between axes differs from the nominal can lead to jamming, and always to mechanical noise even with a predetermined clearance between the flanks of the teeth. If attempts are made to combat difficulties are regards engagement by the use of an excessively large clearance between flanks, as has been done in some known machines, then, because of dynamic oscillations and disparities in the rotary movements of the two wheels or, in the case of hydrostatic machines, because of squeezing pressures, mechanical noise and loading occur whenever a change in the bearing zones on the flanks of the teeth takes place. In the case of geared pumps it is therefore also necessary to accept a rather considerable loss in volumetric efficiency, and a greatly increased pulsation during delivery.